Delta Tau GEO BRICK LV User Manual

Merits of Dual Trigger 
In homing-search moves, it is common practice to use a combination of a homing switch and the index 
channel as the home trigger condition.  The index channel of an encoder, while precise and repeatable, is 
not unique in most applications, because the motor can travel more than one revolution.  The homing 
switch, while unique, is typically not extremely precise or repeatable.  By using a logical combination of 
the two, you can get uniqueness from the switch, and precision and repeatability from the index channel.  
In this scheme, the homing switch is used to select which index channel pulse is used as the home trigger. 

Although the homing switch does not need to be placed extremely accurately in this type of application, it 
is important that its triggering edge remain safely between the same two index channel pulses.  Also, the 
homing switch pulse must be wide enough to always contain at least one index channel pulse. 

Following-Error Triggering 

Sometimes it is desired that a trigger occur when an obstruction such as a hard stop is encountered, as 
when using an end stop for the homing reference.  To support this type of functionality, Turbo PMAC 
permits triggering on a warning following error condition instead of an input flag.  This is sometimes 
called “torque-mode” triggering, because it effectively triggers on a torque level (except for velocity-
mode amplifiers) because output torque command is proportional to following error.  It is also called a 
“torque-limited” mode, because it provides an easy way to create moves that are limited in torque, and 
that stop when the torque limit is reached (as in torque-limited screw driving). 

To enable this torque-mode triggering for Motor xx, set Ixx97 to 3, specifying both following-error 
trigger and software capture (there is no hardware signal to create a hardware capture).  In this mode, the 
trigger for a move-until-trigger is a true state of the warning following-error status bit for the motor.  
Variable Ixx12 sets the warning following-error magnitude for the motor, with units of 1/16 of a count.  
When Turbo PMAC detects that the magnitude of the following error has exceeded this value, it will read 
the present feedback position as the trigger position, then move relative to this position.  

When using torque-mode triggering, it is a good idea to set the integral gain term Ixx33 to 0 to prevent a 
large “charge-up” of the integrator when it hits the hard stop.  It may also be desirable to set the Ixx69 
output limit lower to limit the possible torque directly when the obstruction is reached. 

Note that if the warning following error status bit is true at the start of the move, the trigger will occur 
almost immediately. 

Because the post-trigger move ends at a commanded position expressed relative to the position at the time 
of the trigger, it is necessary for Turbo PMAC to “capture” the position at the time of the trigger.  
Fundamentally, there are two ways of doing this: hardware capture and software capture. 

Hardware Capture 

The Servo ICs of a Turbo PMAC have dedicated registers to latch the encoder counter instantly upon 
receipt of the pre-specified input trigger state.  The latching action occurs entirely in the IC hardware, 
requiring no software action, so the captured position is accurate to the exact count regardless of motor 
speed.  This means that there is no need to slow down the move to get an accurate capture. 

Hardware capture is selected for Motor xx trigger moves by setting Ixx97 to the default value of 0, 
specifying both hardware capture and input triggering.  If hardware capture is selected, the position-loop 
feedback as selected by Ixx03 must come through the encoder counter of a Servo IC.  It must use the 
same hardware channel as the flag set selected by Ixx25.  This means that if you are using dual feedback 
on the motor, the flag set specified by Ixx25 should be the same channel as your position-loop feedback, 
not your velocity-loop feedback.